The objective of this project is to develop new candidate vaccines to elicit protective immune responses to HIV. Although antibodies that neutralize primary isolates of HIV-1 appear during HIV-1 infection, there has been difficulty in eliciting such antibodies by immunization with vaccines comprised of soluble HIV-1 Env antigens. Thus, a new form of antigen which closely resembles the virions present during infection may be more likely to elicit effective neutralizing antibodies to multiple HIV subtypes. We have demonstrated the production of SIV and SHIV virus-like particles (VLPs) containing Gag and Env proteins, and have also developed novel phenotypically-mixed (or chimeric) VLPs containing both influenza hemagglutinin and HIV Env and demonstrated that they are much more effective for mucosal (intranasal) immunization. In this project, we will pursue novel strategies to develop more effective VLP immunogens, and define the immune response to these VLP antigens after mucosal vs. systemic immunization. The specific aims are: (1) We will test the hypothesis that mucosal immune responses elicited by chimeric VLPs containing influenza virus hemagglutinin (HA) are effective in conferring protective immunity in macaques against mucosal challenge. We will also determine whether VLPs containing the HN (hemagglutinin-neuraminidase) protein of human parainfluenza virus will function as effective mucosal vaccines. (2) We will test the hypothesis that more effective VLP immunogens can be developed by incorporation of ligands designed to target the VLPs to dendritic cells, which are highly effective antigen-presenting cells found in various tissues. We will compare three alternative approaches (flt3 ligand, mannose receptor ligand, VEE E2 glycoprotein) to modify the surfaces of VLPs for enhanced targeting to dendritic cells, and determine the effect on immune responses. (3) As the levels of broadly reactive neutralizing antibodies are critical for an effective AIDS vaccine, we will compare several approaches to enhance the immunogenicity of the HIV Env proteins by modification of the structure and conformation of the Env on VLPs. After assessing the results of these aims, we will also determine if combinations of these approaches can be developed to further enhance the efficacy of VLP vaccines. Further, we will interact and collaborate with investigators in the other projects to develop improved VLP formulations to be evaluated in animal models, and considered for advancement to human trials.